1. Field of the Invention
The present invention relates to a transfer system and machine that incorporates the transfer system that transfers an image forming substance from one surface to another surface. More particularly, the invention relates to copying machines, facsimile machines, printer machines and similar image forming apparatuses that include an intermediate transfer element for transferring an image, and in particular, a color image as part of an image forming process.
2. Discussion of the Background
In conventional color image forming apparatuses, a plurality of color toner images are sequentially formed on an image carrier (e.g., a photoconductive element) while being sequentially transferred to the intermediate transfer element one on top of the other (primary transfer). The resulting composite image on the intermediate transfer element is transferred to a sheet or similar transfer medium at a single time (secondary transfer). Such intermediate transfer systems are used in conventional color image forming apparatuses (e.g., a full-color image forming apparatus which reproduces color-separated document images on the basis of using black, cyan, magenta and yellow colored-toner).
In such an image forming apparatus, it is possible that the transfer of toner may fail in certain localities of the image, at the primary and secondary transfer stages. As a result, a full-color image transferred to the sheet or similar transfer medium may be locally lost, or omitted in spots. The local omission of an image occurs with some areas when the image has a substantial area, or occurs as breaks in the case of a line image. In order to address the local omission of an image (i.e., enhancing the transfer ability), various technologies have been proposed in the past and may generally be classified into five groups, as follows.
[I] Reducing Surface Roughness of Intermediate Element
(a) The intermediate element is formed of an elastomer and provided with a particular surface roughness, as disclosed in Japanese Patent Laid-Open Publication No. 3-242667 by way of example. This scheme enhances the close contact of the intermediate element and transfer medium and thereby improves the transferability of toner.
(b) The intermediate element is provided with a particular surface roughness to improve toner transferability, as described in, e.g., Japanese Patent Laid-Open Publication Nos. 63-194272, 4-303869, 4-303872, and 5-193020.
[II] Setting Linear Velocities of Transfer Members
The transfer members are each provided with a particular linear velocity in order to improve the transferability, as described in e.g., Japanese Patent Laid-Open Publication No. 2-213882.
[III] Reducing Pressure at Nip
A nip for image transfer is provided with a particular pressure for improving the transferability, as described in, e.g., Japanese Patent Laid-Open Publication Nos. 1-177063 and 45-284479.
[IV] Reducing Surface Energy of Intermediate Element
(a) The intermediate element is provided with a small degree of wettability in order to enhance the transferability, as disclosed in Japanese Patent Laid-Open Publication Nos. 2-198476 and 2-212867, for example.
(b) The intermediate element has a laminate structure and has the outermost layer formed of a material having a high parting ability, as shown and described in, e.g., Japanese Patent Laid-Open Publication Nos. 62-293270, 5-204255, 5-204257, and 5-303293.
(c) A substance having a high parting ability is fed to the intermediate element in order to enhance the transferability, as disclosed in, e.g., Japanese Laid-Open Publication No. 58-187968.
[V] Removing Toner Film from Intermediate Element
As described in, e.g., Japanese Patent Laid-Open Publication Nos. 5-273893, 5-307344, -313526, and 5-323802, the surface of the intermediate element suffered from toner filming so the surface was ground and refreshed to enhance the transferability.
The local omission of an image at the secondary transfer stage often occurs when using a roller as a secondary transfer mechanism for the following two reasons.
(a) In the case of a full-color image, the toner layer has a substantial thickness. In addition, an intense mechanical adhering force, which is a non-Coulomb's force acting between the intermediate element and the toner, is generated due to the contact pressure attributable to the roller. PA1 (b) When an image forming process is repeated, the toner forms a film on the intermediate element. This toner filming causes an adhering force to act between the intermediate element and the toner. In order to address the above problem, U.S. Pat. No. 5,053,827 entitled "METHOD AND APPARATUS FOR INTERMITTENT CONDITIONING OF A TRANSFER BELT" discloses a conditioning process using a conditioning roller. The contents of U.S. Pat. No. 5,053,827 being incorporated herein by reference.
The conventional schemes [I]-[V] have been proposed independently of each other as measures for enhancing the transferability of toner. Some of the combinations of these schemes are effective while the others are not effective, as determined by experiments.
As to the surface energy of the intermediate belt, a series of extended research and experiments showed that presuming various possible cases, it is extremely difficult to detect the excessive rise of the surface energy in terms of a preselected number of copies.
The technology disclosed in U.S. Pat. No. 5,761,594 entitled "IMAGE FORMING APPARATUS" was introduced to address the above problem (the local omission of the toner image transferred to the intermediate transfer element). Here, the intermediate transfer element has an adhering force acting on toner of a developed image that is greater than or equal to an adhering force acting between the toner and the image carrier.
Further, as a result of subsequent extended experiments and research, it was found by experiments that the toner image may be improperly transferred from the image carrier to the intermediate transfer element under a specific condition even if the above relation between the intermediate transfer element and the image carrier is satisfied. Furthermore, it is possible that the transfer of the toner may locally fail toward the middle portion of a line image when the width of the line image is extremely fine.